1. Field of the Invention
The present invention relates to a communication timing control method, a communication timing control apparatus, a node, and a communication system, more particularly to a method of avoiding data transmission collisions in a communication system including nodes that cannot communicate with each other directly because of intervening obstacles or the like.
2. Description of the Related Art
Known methods by which a plurality of spatially distributed nodes can transmit data without collisions include time division multiple access (TDMA) and carrier sense multiple access (CSMA), the latter including carrier sense multiple access with collision avoidance (CSMA/CA) and carrier sense multiple access with collision detection (CSMA/CD). A discussion of these methods can be found in, for example, %i Waiyaresu LAN Akitekucha (Wireless LAN Architecture), edited by Matsushita and Nakagawa, Kyoritsu Shuppan, 1996, pp. 47, 53-59, and 69 (in Japanese).
In the CSMA methods, a node with data to transmit determines whether other nodes are transmitting by sensing their carrier signals, and waits until no other node is transmitting before transmitting itself. The CSMA methods have the disadvantage of severely restricting the number of channels that can be used simultaneously.
In the TDMA method, different time slots are assigned to different nodes, and each node transmits data in its own assigned time slot. TDMA can provide more simultaneous communication channels than CSMA, but when the set of communicating nodes changes dynamically, an administrative node must reassign the time slots dynamically. A weakness of the TDMA system is that if the administrative node malfunctions, the entire communication system may be brought down. The process by which time slots are assigned dynamically to nodes is also complex, making it difficult to respond promptly to changing conditions. A further problem is that the width of the time slots cannot be changed to accommodate changing amounts of data transmission.
Some of these problems with the TDMA system can be overcome by providing administrative functions in each node and having the nodes assign their own time slots through a negotiation process, but conventional TDMA systems of this type have encountered problems when some of the nodes cannot communicate with each other because of intervening obstacles or the like. Specifically, data transmitted from two nodes that cannot communicate with each other may collide at a third node because the two nodes do not know each other's transmission timing.
It would be desirable to have a flexible and autonomous method and apparatus for controlling communication timing, so that each node could communicate effectively without having to receive timing control instructions from an administrative node, and it would be desirable for this method and apparatus to avoid data collisions despite the presence of obstacles or other obstructions to communication between certain nodes.